<p>Bystanders in cyberbullying play a crucial role by either defending victims, withdrawing from the incident, or reinforcing bullying behaviors. This study focused on three specific bystander roles in cyberbullying—defender, outsider, and reinforcer—and examined their associations with individual (self-control, belief in a just world), familial (parental support), and social (social presence) factors. Data were obtained from 1,556 valid responses collected in four public middle schools in southern China. Results indicated that girls were more likely than boys to engage as defenders, outsiders, and reinforcers, whereas no significant differences were observed between local and migrant students in role involvement. Multi-group structural equation modeling further demonstrated that the paths from self-control to defender and from belief in a just world to defender differed significantly by gender. In terms of migration status, significant group differences were found in the paths from self-control to outsider, self-control to reinforcer, and belief in a just world to reinforcer. Overall, these findings underscore the need to account for both gender and migration status when developing targeted interventions aimed at fostering constructive bystander behaviors in cyberbullying contexts.</p>